MUMBAI: Through the haze, experts see a smokescreen – the civic administration’s attempt to monitor air quality at construction sites, a key source of pollution in a city where poor air has, for the first time, sounded alarm bells.
In May last year, the BMC mandated that developers and building contractors install monitors that display air quality levels (Air Quality Index or AQI) as well as levels of particulate matter. This was in response to litigation on Mumbai’s poor air quality being heard in the Bombay High Court.
The BMC told the court last month that of the 1,954 construction sites at present, 1,292 sensor-based air quality monitor installations have been completed, and 110 installations were underway.
But a site visit by Hindustan Times to ten private and government construction sites revealed data that appears misleading. Readings at sites just a few metres apart varied widely, especially in PM10 and PM2.5 levels displayed on the monitors.
In Wadala, for instance, the site of a private developer recorded PM₁₀ at 82.1 and PM₂.₅ at 19.0, while barely 100 metres away, another site near the Wadala Monorail Station (Anik Depot) showed significantly higher levels, with PM₂.₅ at 94 and PM₁₀ at 139.
Another nearby RMC plant recorded comparatively lower values, with PM₂.₅ at 34 and PM₁₀ at 50, despite operating within the same micro-environment.
Similar variations were noted in Dadar (Shivaji Park), where a private construction site recorded PM₂.₅ at 36 and PM₁₀ at 37, while an adjacent project reported much higher readings of PM₂.₅ at 100 and PM₁₀ at 114 . These sites are located within just 100 metres of each other.
The choice of monitors is not left to the project developer. They have to choose from a list of 15 manufacturers shortlisted by the BMC.
How do developers explain these glaring discrepancies?
Mohammad Hamza, managing director of Engineering and Environmental Solutions Pvt Ltd, said variations in readings are expected. “Most of these compact monitors work on the principle of light scattering. Differences in sensor design, calibration approaches, and environmental correction factors across manufacturers can lead to some variation in PM10 and PM2.5 values,” he said.
Experts said the variation in PM10 and PM2.5 levels displayed on these monitors is not surprising. Simply put, the sensors lack the precision required for scientific-grade measurement. “They produce errors even when dust settles on them – and these are construction sites!”
Rakesh Kumar, environmental scientist and president of the Society for Indoor Environment, said “Multiple layers of pollutants on the sensors can lead to either erroneous measurements or malfunctioning of the sensors. Natural factors like humidity and temperature also interfere with accurate readings.”
Another environmentalist said the PM 2.5 levels at one construction site in Andheri was registering as “2”. “PM 2.5 levels, not even in the remotest corners, can be single digit,” he pointed out.
“Choosing one from 15 different sensors is a problematic approach. The civic body should have given two to choose from… this exercise is hogwash!”
Advocate Trivankumar Karnani, founder of the MNCDF Citizen Welfare Forum, pointed out that the high court has flagged defective and inaccurate air quality monitors. “Mumbai’s air quality monitoring is insufficient, inconsistent, and fails the test of accountability. When civic authorities cannot even measure pollution with accuracy, mitigation is a hollow promise. The court’s decision to form a panel is a welcome step, but it must translate into enforceable norms and real-time accountability,” he said.
Arun Kate, chief engineer, environment & climate change department, BMC, said, “These are low-cost sensors designed to capture source-level readings rather than ambient air quality. They are highly sensitive and can be triggered even by activities such as the burning of incense sticks. Variations in readings may therefore depend on the specific activities taking place at individual sites, even if they are adjacent to one another,” said Kate.
“We will examine the discrepancies and the concerns citizens have raised, on a case-by-case basis and allow some time for the system to stabilise. The department will review instrument calibration, correct positioning, and maintenance practices as part of this process.”
Ex-judges to monitor compliance
The Bombay High Court recently constituted a high-power committee (HPC) comprising Amjad A Sayed, former chief justice of the Himachal Pradesh High Court and Anuja Prabhudessai, former judge of the Bombay High Court.
The court was hearing a suo motu PIL initiated in 2023, along with several intervention applications filed following the recent deterioration in Mumbai’s air quality.
The court, on January 29, said the committee will supervise compliance with earlier orders passed by the high court and will make recommendations to the authorities for formulation of immediate, medium- and long-term measures to combat air pollution in Mumbai.
Box: Why size matters
While PM2.5 refers to fine particulate matter 2.5 micrometres in diametre (30 times less than the width of a human hair), PM10 is less than 10 micrometres in diameter (five times less than the width of a human hair. Construction dust accounts for much of the PM10 in Mumbai’s air. Once inhaled, these particles can affect the heart and lungs and cause serious health effects.